This invention relates generally to infrared image tubes and particularly relates to a cold cathode for such a tube having a high ratio of emitted free electrons to injected photons.
Modern image sensing devices or image tubes have two basic constructions. One type of image tube is based on the conventional silicon technology. The other construction makes use of the new possibilities based on semi-conductor compounds of Group III - V coupled with the electron emission based on negative electron affinity.
The tubes based on silicon technology feature silicon diode arrays. The diodes in turn must be read out with an electron beam. However, these devices suffer from defects of the diode structure which result in differences in the performance of individual diodes and their differences in resolution.
Recently, charge-coupled devices have been applied to provide better resolution. This has been explained in a paper by Sequin et al entitled "A Charge-Coupled Area Image Sensor and Frame Store" which appears in IEEE Transactions on Electron Devices, Volume ED-20, No. 3, March 1973, pages 244 - 252. This device is a three-phase system, that is, the readout is carried out in three phases and provides better resolution. The device built was provided with 64 .times. 106 imaging cells.
In order to readout the image the line shifting requires a long integration time. Therefore, for live television the device must be operated in a frame transfer mode, that is, an entire frame is transferred. This device features an active cathode which has the advantage of speed. The readout is effected in a simultaneous process of picture projection by impingement of the secondary electrons on a video screen.
Silicon may also be used in a secondary electron emission mode. However, such a device suffers from high dark current, that is a current in the absence of light and this is due to the properties of the material.
Therefore, for high speed imaging at low light levels, the negative electron affinity concept is very promising. This has been reported by Simon in IEEE Spectrum, December 1972, pages 74 - 78. As stated in the paper, photoemissive sensitivity of negative electron affinity cathodes has dramatically improved since 1958. The values of current in micro-amperes per lumen of instant radiation have increased during this period by a factor of 5 - 6.
Numerous proposals have been made in the past to increase the yield or efficiency of infrared converter tubes. It has been shown that secondary emission from cold cathodes of the Group III - V can be orders of magnitude higher than that for Magnesium oxide. Such cold cathodes may, for example, consist of GaP. It also has been proposed to use heterojunctions of Germanium, Zinc selenide, and P-type Gallium arsenide. For example, one may use Germanium for the infrared emission and Zinc selenide as a junction material. The reason is that there is a close match of the Zn Se lattice to Germanium. In this case, a P-type Gallium arsenide layer is the electron-emitting surface.
Such structures have limited value. They have been used to analyze theoretically the models because neither metallurgical nor technical solutions exist for the required high perfection growths of these heterojunctions with good lattice match.
A paper by Schade entitled "Efficient Electron Emission From GaAs-Al.sub.1-x Ga.sub.x As Optoelectronic Cold-Cathode Structures," which appears in Applied Physics Letters, Volume 18, No. 10, May 15, 1971, pages 413 - 414, deals with semi-conductor cold cathodes and proposed junctions of Al.sub.1-x Ga.sub.x As silicon doped covered with a top layer of GaAs Zinc doped and activated with Cesium and oxygen.
A semi-conductor surface as explained in some of the papers referred to when covered with Cesium or Cesium-oxide will show a negative electron affinity. This is particularly true when care is taken to place the bottom of the conduction band at an energy level higher than that of the vacuum potential. This is so when P-type doping is used. High doping density additionally causes the conduction band depression to increase and the band bending distance to decrease. As a result this distance can be less than the escape depth of the electron into the vacuum.
The surface barrier normally present at the outer surface layer can be reduced by the dipole layer of Cesium or Cesium-oxide.
It is an object of the present invention to produce a cold cathode for an infrared converter or image tube providing high efficiency by appropriately using heterojunctions.
Another object of the present invention is to provide a cold cathode of the type discussed which combines the relatively high quantum efficiency of Gallium arsenide cathodes in the near infrared with the high injection efficiency of P-N heterojunctions.
A further object of the present invention is to provide such a cathode by liquid phase epitaxy.